The invention relates to the general field of MEMS structures with particular reference to cantilever beams.
MEMS (micro electromechanical systems) sensors and actuators, such as accelerometers, pressure sensors, and gyroscopes are manufactured using either a bulk micromachining process or a surface micromachining process. xe2x80x9cBulkxe2x80x9d micromachining refers to structures formed by deep anisotropic etching. xe2x80x9cSurfacexe2x80x9d micromachining refers to structures formed from thin film layers deposited or grown on the surface of a substrate. Surface micromachining has advantages over the previous bulk micromachining process of fabricating IC sensors and actuators because it permits smaller devices and may be integrated with other circuits on an IC (integrated circuit). One form of bulk micro-machining typically involves etching in a silicon substrate deep trenches between 10 microns to 100 microns deep. The resulting silicon structures (called xe2x80x9cbeamsxe2x80x9d) are partially released (i.e., detached) from the silicon substrate by known processes such as wet or dry etching. This deep trench technology is described, for example, in Klaassen, et al. xe2x80x9cFusion Bonding and Deep Reactive Ion Etching: A New Technology for Microstructuresxe2x80x9d, Transducers ""95, Stockholm, Sweden, 1995. The contents of this article are incorporated herein by reference.
A variety of methods that have been discussed in the literature have been devised for fabricating micromachined structures such as accelerometers. However, most such processes require multiple masking steps, wafer-to-wafer bonding, or the use of wet chemistry. It has been found, however, that the use of such multiple masks and bonding techniques can introduce alignment errors, which reduce yield and increase device cost, making such processes unsuitable for submicron structures.
A routine search of the prior art was performed with the following references of interest being found:
U.S. Pat. No. 5,930,595 (Isolation process for surface micromachined sensors and actuators) discusses a method of fabricating MEMS sensors/actuators using a process wherein deep trenches are etched and released beams formed by using oxide spacer to protect beam sidewall. The key feature of this patent is that it provides a novel method of forming trenches which are filled with isolation oxide so as to form silicon islands on three sides while the fourth side is connected to the sensor/actuator beams. Recently, another patent application has been filed in IME, namely, xe2x80x9cA High Aspect Ratio Trench Isolation Process for Surface Micromachined Sensor and Actuators (PAT00-005/MEMS001)xe2x80x9d which uses a novel process to form an isolation island that can be used in fabrication of MEMS sensors/actuators.
U.S. Pat. No. 5,563,343 describes a method of fabricating accelerometers utilizing a modified version of the Single Crystal Reactive Etching. And Metallization (SCREAM) process which is also described in U.S. application Ser. No. 08/013,319, filed Feb. 5, 1993. As stated in that application, the SCREAM-I process is a single mask, single wafer, dry etch process which uses optical lithography for fabricating submicron micro-electromechanical devices. In that process, a silicon dioxide layer is deposited on a single crystal silicon wafer, this oxide layer serving as the single etch mask throughout the process. Photolithography is used to pattern a resist, and then dry etching, such as magnetron ion etching, is used to transfer the pattern of the accelerometer structure into the oxide. Once the resist material is removed, the patterned oxide masks the silicon substrate to allow a deep vertical silicon RIE (reactive ion etching) on exposed surfaces to produce trenches having predominately vertical side walls and which define the desired structure.
Next, a conformal coating of PECVD oxide is deposited for protecting the side walls of the trenches during the following release etch. The trench bottom oxide is removed within an isotropic RIE, and a second deep silicon trench etch deepens the trenches to expose the sidewall silicon underneath the deposited side wall oxide. The exposed silicon underneath the defined structure is etched away, using an isotropic dry etch such as an SF6 etch to release the structure and leave cantilevered beams and fingers over the remaining substrate. In the SCREAM-I process, aluminium is deposited by sputtering to coat the sidewall of the released beams and fingers to thereby form the capacitor plates for the accelerometer.
In U.S. Pat. No. 6,035,714, a high sensitivity, Z-axis capacitive micro-accelerometer having stiff sense/feedback electrodes and a method of its manufacture are provided. The micro-accelerometer is manufactured out of a single silicon wafer and has a sili-con-wafer-thick proofmass, small and controllable damping, large capacitance variation and can be operated in a force-rebalanced control loop. The multiple stiffened electrodes have embedded therein-amping holes to facilitate both force-rebalanced operation of the device and controlling of the damping factor. Using the whole silicon wafer to form the thick large proofmass and using the thin sacrificial layer to form a narrow uniform capacitor air gap over a large area provide large capacitance sensitivity. The structure of the micro-accelerometer is symmetric and thus results in low cross-axis sensitivity.
In U.S. Pat. No. 5,660,680, a method of forming polysilicon structures using silicon trenches with partially trench-filled oxide as molds has been described. The oxide layer acts as the sacrificial layer to release the polysilicon structures.
BOSCH Polysilicon (Epi-poly) process: This process makes use of thick epitaxial polysilicon (20-30 microns) grown on a silicon substrate. This poly layer is then used in forming beams of various depths for forming MEMS structures. This process uses an epi reactor and hence is quite expensive. For thick poly, residual stress is still a potential issue.
Additional references of interest were:
U.S. Pat. No. 6,133,670 (Rodgers) shows a poly beam (finger) in a MEMS device. In U.S. Pat. No. 6,175,170 B1, Kota et al. show another poly finger MEMS device and process while, in U.S. Pat. No. 6,171,881 B1, Fujii shows another MEMS device.
It has been an object of the present invention to provide a cost-effective process for manufacturing IC sensors and/or actuators that completely electrically isolates the sensor beams from the substrate that supports them.
Another object of the present invention has been to provide a process for manufacturing IC sensors and/or actuators that have low parasitic capacitance.
Yet another object of the present invention has been to provide a process for manufacturing IC sensors and/or actuators that is compatible with CMOS processes.
These objects have been achieved by providing a process which makes use of polysilicon beam as the structural material instead of single crystal silicon for the fabrication of MEMS sensors/actuators. The invention describes the process for fabricating suspended polysilicon beams by using deep trenches etched into silicon substrate as molds to form polysilicon beams. The polysilicon beams are subsequently released by isotropically etching away the silicon surrounding the polysilicon beams. This results in free standing polysilicon members, which form the MEMS structures. In addition to the general process, three approaches to making electrical contact to the beams are presented.